foo.c: In function 'main':
foo.c:15:17: error: 'sum' undeclared (first use in this function)
   15 |                 sum+=a*b;
      |                 ^~~
foo.c:15:17: note: each undeclared identifier is reported only once for each function it appears in

[Hydro](https://hydro.ac)提供评测服务